Site.login(username,password,Starturl) returns null

Question

        String startURL = '/PageALanding';
        String retUrl = 
        System.currentPageReference().getParameters().get('retURL');
        system.debug('+++++++++++++++++++++++'+returl);

        if(retUrl != null){
            return Site.login(username, password, retUrl);
        }


        PageReference pr = Site.login(username, password, startUrl);
        System.debug('sjhdfhsdf'+pr);

        if (pr == null){
            loginError = true;
        }

        return pr;